FIG. 1 shows an example of a boiler 1 having an evaporator 2 defined by walls 3 (tubed walls, preferably finned tubed wall); the walls 3 define a chamber 4 and the bottom of the walls 3 defines a hopper 5.
One or also more than one walls 3 carry a firing system 6 comprising a fan for an oxidizer like air and a fuel supply 8 for coal, oil, gas, etc.
The tubed walls 3 are connected to inlet headers 9 and outlet headers 10; water is collected at the inlet headers 9 and is distributed through the tubes of the tubed walls 3 and, after passing through the tubed walls 3, steam (or a mixture of steam and water or steam containing some water to a low extent) is collected at the outer headers 10. The headers 9 and 10 are outside of the chamber 4. Naturally also other types of evaporators are possible.
Above the evaporator 2, the boiler 1 has a duct 12 that houses in series, from the bottom to the top, a superheater 13 for heating the steam directed to a high pressure user (like for example a high pressure turbine 13a of a power plant) and a reheater 14 for heating the steam discharged from the high pressure user and directed to a medium or low pressure user (like for example a medium or low pressure turbine 14a of a power plant).
The superheater 13 includes heat exchanging components having tubed heat exchanging surfaces 16 connected to inlet headers 17 and outlet headers 18; for example the tubed heat exchanging surfaces 16 can be tubed coils or tubed panels.
The attached FIGURE shows an example of a superheater 13 including three heat exchanging components each having tubed heat exchanging surfaces 16, inlet header 17 and outlet header 18.
The reheater 14 has a structure similar to the structure of the superheater 13.
The reheater 14 includes heat exchanging components that comprise tubed heat exchanging surfaces 16, such as tubed coils or tubed panels. The tubed heat exchanging surfaces 16 are connected to inlet headers 17 and outlet headers 18.
The attached FIGURE shows an example of a reheater 14 including two heat exchanging components each having tubed heat exchanging surfaces 16, inlet header 17 and outlet header 18.
Above the reheater 14 there is provided an economizer 20, to pre-heat water coming from a feedwater source 20a and directed to the evaporator 2. The economizer 20 is also provided with inlet headers and outlet headers.
In the duct 12, downstream the economizer 20, there are typically installed a catalyzer 21 (if needed according to the emission requirements) for reducing the NOx content of the flue gas, a preheater 22 for preheating air that is supplied into the chamber 4 for combustion of the fuel, a dust removal unit 23 such as a filter or electrostatic precipitator for solid particles removal from the flue gas; in some cases a damper 24 for regulating the opening of the flue gas duct 12 and a fan 7 for transportation of the flue gas to the stack 34 can also be provided.
In some cases, the economiser 20 can be separated in two parts, one upstream the catalyzer 21 and one downstream the catalyzer 21.
During operation, water passes through the economizer 20 where it starts heating and then it is supplied through the headers 9 to the tubed walls 3. While passing through the tubed walls 3 water evaporates, generating steam that is collected at the headers 10 and is directed (through a separating system 25 to remove possible liquid droplets) to the super heater 13 via the headers 18a. The first stage of the super heater 13 can either be the upper (vertical) boiler enclosure wall or the internal hanger tubes ending in the first super heater bundle.
Downstream of the superheater 13, superheated steam is directed to the high pressure turbine 13a for example of a power plant or for other high pressure user or to the reheater 14 inlet via the high pressure bypass valve 26.
Steam from the high pressure turbine 13a or other high pressure user is collected at the inlet header 17 of the reheater 14 and, after passing through the reheater 14 it is collected in the outlet header 18 from which it is directed to the medium or low pressure turbine 14a or medium or low pressure user or via the low pressure bypass valve 27 to the condenser 35 provided downstream of the steam turbine.
Liquid droplets collected at the separating system 25 are directed back through the recirculation pump 29 to the economizer 20.
During shut down the firing system 6 is stopped, the high pressure turbine 13a and the medium or low pressure turbine 14a are disconnected and the valves 26 and 27 are closed.
For this reason, the steam passing through the superheater 13 and reheater 14 is stopped, i.e. there is no further steam flow within the heating surfaces 16 of the superheater 13 and the reheater 14.
Nevertheless, during shut down air keep circulating through the chamber 4, this is due for example to purging or natural draft. For example, often the fan 7 operates for maintaining an underpressure inside the boiler enclosure also during shut down. This causes an air flow at temperature lower than the temperature of the steam within the superheater 13 and reheater 14.
The flow increases the cooling of the steam contained within the tubed heat exchanging surfaces 16 of the superheater 13 and reheater 14. This cooling can be large, because the thickness of the surfaces of the tubed heat exchanging surfaces 16 is usually small, such that the thermal storage capacity of the tube walls is low.
In contrast, the steam contained within the headers 17, 18 only undergoes a very limited cooling.
In fact, the headers 17, 18 have a large wall thickness and therefore they also have a large thermal storage capacity.
In addition, the headers 17, 18 are insulated such that substantial cooling from the outside of the headers 17, 18 is prevented; moreover, since there is no steam flow inside the headers 17, 18, no substantial cooling from the inside of the headers 17, 18 occurs.
As a consequence, the temperature of the steam and of the header 17, 18 of the reheater 14 and superheater 13 (i.e. of the material of the header 17, 18) will decrease only with a very small gradient (i.e. the temperature of this steam slowly decreases), but the temperature of the steam contained in the tubed heat exchanging surfaces 16 of the reheater 14 and superheater 13 sensibly drops.
When the boiler 1 is start up again after shut down, the firing system 6 is started and the high pressure bypass valve 26 and the low pressure bypass valve 27 are opened.
Opening the high pressure bypass valve 26 and the low pressure bypass valve 27 causes steam circulation through the tubed heat exchanging surfaces 16 and the headers 17, 18 of the superheater 13 and the reheater 14. This circulation causes steam at a low temperature (because it was contained within the tubed heat exchanging surfaces 16 during shut down) to pass through the headers 17, 18 that have a much higher temperature.
This circulation thus causes thermal stress of the material of the header 17, 18 and possibly a reduction of the lifetime.